Intracerebral inoculation of neonatal beagles with the Schmidt- Ruppin strain of the Rous Sarcoma Virus induces gliomas or intracranial sarcomas with 100 percent incidence. These autochthonous, experimental brain tumors are highly similar to spontaneous brain tumors in man at both the light and electronmicroscopic level. The purpose of this research project is to: a) study in detail the microvasculature of these tumors to delineate possible structural bases for altered permeability of brain barrier mechanisms in such tumors, b) study the intercellular contacts between the neoplastic glial cells in comparison to normal neuroglia and c) evaluate the hypothesis that the neoplastic glial cells arise from and are products of viral transformation of normal glial precursors in the subependymal cell-plate. Early events in viral transformation of primitive glial precursors will be studied, and the development of the neoplastic glial cell will be compared to normal cellular differentiation.